Method of and circuit for accepting a sample of an input signal to be used to calculate parameters for a predistortion circuit in an integrated circut

ABSTRACT

A method of accepting a sample of an input signal to be used to calculate parameters for a predistortion circuit in an integrated circuit is disclosed. The method comprises accumulating data associated with an input signal over a period of time; detecting the sample of the input signal at a predetermined time; comparing the sample of the input signal to the accumulated data; and determining whether the sample of the input signal is acceptable to be used to calculate parameters for the predistortion circuit. A circuit for accepting a sample of an input signal to be used to calculate parameters for a predistortion circuit in an integrated circuit is also disclosed.

FIELD OF THE INVENTION

The present invention relates generally to integrated circuits, and inparticular, to a method of and circuit for accepting a sample of aninput signal to be used to calculate parameters for a predistortioncircuit in an integrated circuit.

BACKGROUND OF THE INVENTION

Integrated circuits are an integral part of any electronic device. Avariety of integrated circuits are often used together to enable theoperation of the electronic device. While integrated circuits aretypically designed for a particular application, certain integratedcircuits may have configurable logic. For example, an applicationspecific integrated circuit (ASIC) may have portions comprisingconfigurable logic. Another type of integrated circuit which hasconfigurable logic is a programmable logic device (PLD). A programmablelogic device is designed to be user-programmable so that users mayimplement logic designs of their choices. One type of programmable logicdevice is a Complex Programmable Logic Device (CPLD). A CPLD includestwo or more “function blocks” having a two-level AND/OR structureconnected together and to input/output (I/O) resources by aninterconnect switch matrix. Another type of programmable logic device isa field programmable gate array (FPGA). In a typical FPGA, an array ofconfigurable logic blocks (CLBs) is coupled to programmable input/outputblocks (IOBs). The CLBs and IOBs are interconnected by a hierarchy ofprogrammable routing resources. For both of these types of programmablelogic devices, the functionality of the device is controlled byconfiguration data bits of a configuration bitstream provided to thedevice for that purpose.

Integrated circuits may be used in arrangements for transmitting data.Because data transmitted using a power amplifier (PA) may be distorted,a predistortion circuit may be employed. The purpose of predistortion isto negate the non-linear effects of a power amplifier. Predistortion isachieved by applying a non-linear filter to the signal prior totransmission. The predistortion circuit acts on the transmitted data tocancel the distortion in the PA by implementing an inverse model of theamplifier. The predistortion circuit may be a function having a vectorof parameters applied to the sequence of transmitted data. However,implementing a predistortion circuit in a device having programmablelogic poses a number of problems.

SUMMARY OF THE INVENTION

A method of accepting a sample of an input signal to be used tocalculate parameters for a predistortion circuit in an integratedcircuit is disclosed. The method comprises accumulating data associatedwith an input signal over a period of time; detecting the sample of theinput signal at a predetermined time; comparing the sample of the inputsignal to the accumulated data; and determining whether the sample ofthe input signal is acceptable to be used to calculate parameters forthe predistortion circuit. The method may further comprise establishcriteria for determining whether the sample of the input signal isacceptable to be used to calculate parameters for the predistortioncircuit, wherein the criteria comprises a plurality of tests.Determining whether the sample of the input signal is acceptable maycomprises determining whether at least one test or all tests of theplurality of tests applied to the sample of the input signal are passed.Accumulating data associated with an input signal over a period of timemay comprise storing information related to the input signal in amemory, wherein detected amplitudes of the input signal are used asaddresses for storing counts associated with the detected amplitudes.

According to an alternate embodiment, a method of accepting a sample ofan input signal to be used to calculate parameters for a predistortioncircuit in an integrated circuit comprises establishing criteria fordetermining whether data associated with the input signal is acceptableto be used to calculate parameters for the predistortion circuit;accumulating power measurements for the input signal over a period oftime; taking a power measurement for a sample of the input signal inresponse to a capture trigger; comparing the power measurement for theinput signal taken in response to the capture trigger to the accumulatedpower measurements; and determining whether the sample of the inputsignal is acceptable to be used to calculate parameters for thepredistortion circuit. Various criteria are described.

A circuit for accepting a sample of an input signal to be used tocalculate parameters for a predistortion circuit in an integratedcircuit is also disclosed. The circuit comprises a predistortion circuitcoupled to receive the input signal; a first memory for storing dataassociated with the input signal and accumulated over a period of time;a second memory storing data associated with a sample of the inputsignal; and a control circuit coupled to the first memory and the secondmemory, the control circuit comparing the data in the second memory tothe data associated with the input signal stored in the first memory fordetermining whether the sample of the input signal is acceptable to beused to calculate parameters for the predistortion circuit. The circuitmay further comprise an estimator circuit coupled to the sample capturebuffers, wherein the estimator circuit generates parameters for thepredistortion circuit based upon the output of the predistortion circuitand the output of the power amplifier received at the integratedcircuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a device having configurable logicaccording to an embodiment the present invention;

FIG. 2 is a block diagram of a configurable logic element of the deviceof FIG. 1 according to an embodiment of the present invention;

FIG. 3 is a block diagram of an integrated circuit having a circuit forreducing distortion at the output of a power amplifier according to anembodiment of the present invention;

FIG. 4 is a block diagram of an integrated circuit having a circuit forreducing distortion at the output of a power amplifier and an embeddedprocessor according to an embodiment of the present invention;

FIG. 5 is a block diagram of an integrated circuit having a circuit forreducing distortion at the output of a power amplifier and an embeddedprocessor according to an alternate embodiment of the present invention;

FIG. 6 is a block diagram of an integrated circuit having a circuit forreducing distortion at the output of a power amplifier and circuits foradapting coefficients according to an embodiment of the presentinvention;

FIG. 7 is a graph showing adjacent channel power as a function oftransmitted power for a plurality of parameter sets having coefficientsfor a predistortion circuit according to an embodiment of the presentinvention;

FIG. 8 is a chart showing the capture of input signals over a period oftime according to an embodiment of the present invention;

FIG. 9 is a block diagram of a circuit for accepting a sample of aninput signal to be used to calculate parameters for a predistortioncircuit of an integrated circuit according to an embodiment of thepresent invention;

FIG. 10 is a block diagram of a random access memory for storing datarelated to the input signal over a period of time according to anembodiment of the present invention;

FIG. 11 is a block diagram of a random access memory for storing datarelated to input signals captured in response a capture trigger signalaccording to an embodiment of the present invention;

FIG. 12 is a block diagram of a circuit for generating parameters for apredistortion circuit according to an embodiment of the presentinvention;

FIG. 13 is a block diagram of the signal correction circuit 1202 of FIG.12 according to an embodiment of the present invention;

FIG. 14 is a block diagram of the amplitude alignment circuit 1302 ofFIG. 13 according to an embodiment of the present invention;

FIG. 15 is a block diagram of the delay alignment circuit 1304 of FIG.13 according to an embodiment of the present invention;

FIG. 16 is a block diagram of the edge detection circuit 1502 of FIG. 15according to an embodiment of the present invention;

FIG. 17 is a block diagram of the frequency offset correction circuit1306 of FIG. 13 according to an embodiment of the present invention;

FIG. 18 is a flow chart showing criteria for selecting a sample of aninput signal to calculate parameters for a predistortion circuitaccording to an embodiment of the present invention;

FIG. 19 is a flow chart showing a method of determining whether a sampleof an input signal is acceptable to be used to calculate parameters fora predistortion circuit according to an embodiment the presentinvention; and

FIG. 20 is a flow chart showing a method of generating parameters for apredistortion circuit according to an embodiment the present invention.

DETAILED DESCRIPTION

Turning first to FIG. 1, a block diagram of a device having configurablelogic according to an embodiment the present invention is shown. Thedevice of FIG. 1 comprises an FPGA architecture 100 having a largenumber of different programmable tiles including multi-gigabittransceivers (MGTs 101), configurable logic blocks (CLBs 102), randomaccess memory blocks (BRAMs 103), input/output blocks (IOBs 104),configuration and clocking logic (CONFIG/CLOCKS 105), digital signalprocessing blocks (DSPs 106), specialized input/output blocks (I/O 107)(e.g., configuration ports and clock ports), and other programmablelogic 108 such as digital clock managers, analog-to-digital converters,system monitoring logic, and so forth. Some FPGAs also include dedicatedprocessor blocks (PROC 110).

In some FPGAs, each programmable tile includes a programmableinterconnect element (INT 111) having standardized connections to andfrom a corresponding interconnect element in each adjacent tile.Therefore, the programmable interconnect elements taken togetherimplement the programmable interconnect structure for the illustratedFPGA. The programmable interconnect element (INT 111) also includes theconnections to and from the programmable logic element within the sametile, as shown by the examples included at the top of FIG. 1.

For example, a CLB 102 may include a configurable logic element (CLE112) that may be programmed to implement user logic plus a singleprogrammable interconnect element (INT 111). A BRAM 103 may include aBRAM logic element (BRL 113) in addition to one or more programmableinterconnect elements. The BRAM comprises dedicated memory separate fromthe distributed RAM of a configuration logic block. Typically, thenumber of interconnect elements included in a tile depends on the heightof the tile. In the pictured embodiment, a BRAM tile has the same heightas four CLBs, but other numbers (e.g., five) may also be used. A DSPtile 106 may include a DSP logic element (DSPL 114) in addition to anappropriate number of programmable interconnect elements. An IOB 104 mayinclude, for example, two instances of an input/output logic element(IOL 115) in addition to one instance of the programmable interconnectelement (INT 111). The location of connections of the device iscontrolled by configuration data bits of a configuration bitstreamprovided to the device for that purpose. As will be described in moredetail below, the programmable interconnects, in response to bits of aconfiguration bitstream, enable connections comprising interconnectlines to be used to couple the various signals to the circuitsimplemented in programmable logic, or other circuits such as BRAMs orthe processor.

In the pictured embodiment, a columnar area near the center of the die(shown shaded in FIG. 1) is used for configuration, clock, and othercontrol logic. Horizontal areas 109 extending from this column are usedto distribute the clocks and configuration signals across the breadth ofthe FPGA. Some FPGAs utilizing the architecture illustrated in FIG. 1include additional logic blocks that disrupt the regular columnarstructure making up a large part of the FPGA. The additional logicblocks may be programmable blocks and/or dedicated logic. For example,the processor block PROC 110 shown in FIG. 1 spans several columns ofCLBs and BRAMs.

Note that FIG. 1 is intended to illustrate only an exemplary FPGAarchitecture. The numbers of logic blocks in a column, the relativewidths of the columns, the number and order of columns, the types oflogic blocks included in the columns, the relative sizes of the logicblocks, and the interconnect/logic implementations included at the topof FIG. 1 are purely exemplary. For example, in an actual FPGA more thanone adjacent column of CLBs is typically included wherever the CLBsappear, to facilitate the efficient implementation of user logic. Thecircuits and methods of the present invention may be implemented in thePLD of FIG. 1, or in any other suitable device, including any type ofintegrated circuit having programmable logic.

Turning now to FIG. 2, a block diagram of a configurable logic elementof the device of FIG. 1 according to an embodiment of the presentinvention is shown. In particular, FIG. 2 illustrates in simplified forma configurable logic element of a configuration logic block 102 ofFIG. 1. The configurable logic element shown comprises two similarslices, where each slice comprises a pair of function generators.However, the configurable logic element may comprise more slices, suchas four slices, for example. Each function generator may function in anyof several modes depending upon the configuration data in theconfiguration memory elements M1-M14. When in RAM mode, input data issupplied by input terminals DI_1 and DI_2 to the data input (DI)terminal of the associated function generator. Each function generator,which may comprise a lookup table, provides an output signal to anassociated multiplexer, which selects between the output signal of thefunction generator and an associated register direct input signalReg_DI_1 or Reg_DI_2 from the programmable interconnect element. Thus,each function generator may be optionally bypassed. When in a look-uptable mode, each function generator implemented as a look-up table hasfour data input signals IN0-IN3. Slice 1 comprises a function generatorimplemented as a LUT 202 coupled to a multiplexer 204. In particular,the LUT 202 receives 4 input signals which are decoded to generate anoutput D1 associated with data stored in the LUT at the addressdesignated by the input signals. The multiplexer 204 is adapted toreceive the output of LUT 202 and a registered value of Reg_DI_1. Theoutput of the multiplexer 204 is coupled to a register 206 whichgenerates an output Q1.

A Write Control Circuit 208 is coupled to receive RAM control signalsand generate signals to control the LUT 202. In addition to a data input(DI) coupled to receive DI_1 and conventional read and write controlsignals coupled to a read enable input (R) and a write enable input (W),respectively, the LUT 202 comprises a partial reset input (RST) forreceiving a partial reset signal, and an initial state input (IS) forreceiving an initial state signal. Such resetting of the memory elementsenables resetting the LUT memory cells during a partial reconfigurationof a programmable logic device, including partial reconfiguration of adevice during operation. One advantage of resetting LUT memory elementsof a device during partial reconfiguration is that it is not necessaryto cycle through the required clock cycles to set the correct data afterthe partial reconfiguration. Similarly, slice 1 comprises a functiongenerator implemented as a LUT 210 coupled to a multiplexer 212. The LUT210 is adapted to receive input signals IN4-IN7, while the multiplexer212 is coupled to receive the output D2 of the LUT 210 and theregistered input value Reg_DI_2. The output of the multiplexer 212 iscoupled to a register 214 which generates an output Q2. The writecontrol circuit 208 also generates a partial reset signal and an initialstate signal for selectively resetting or setting one or more of thebits of the LUT 210.

Similarly, slice 2 comprises a function generator implemented as a LUT222 coupled to a multiplexer 224. The LUT 222 is adapted to receiveinput signals IN8-IN11, while the multiplexer 224 is coupled to receivethe output of the LUT 222 and a registered input value Reg_DI_3. Theoutput of the multiplexer 224 is coupled to a register 226 whichgenerates an output Q3. A Write Control Circuit 228 is coupled toreceive RAM control signals and generate signals to control the LUT 222.In particular, input signals IN8-IN11 are decoded to generate an outputD3 associated with data stored in the LUT at the address designated bythe input signals. The LUT 222 comprises a partial reset input (RST) forreceiving a partial reset signal, and an initial state input (IS) forreceiving an initial state signal. Similarly, slice 2 comprises afunction generator implemented as a LUT 230 coupled to a multiplexer232. The LUT 230 is adapted to receive input signals IN12-IN15, whilethe multiplexer 232 is coupled to receive the output D4 of the LUT 230and a registered input value Reg_DI_4. The output of the multiplexer 232is coupled to a register 234 which generates an output Q4. The writecontrol circuit 228 also generates a partial reset signal and an initialstate signal for selectively resetting or setting one or more of thebits of the LUT 230.

Turning now to FIG. 3, a block diagram of an integrated circuit having acircuit for reducing distortion at the output of a power amplifieraccording to an embodiment of the present invention is shown. Inparticular, an integrated circuit 301 is coupled to a power amplifier302. As will be described in more detail below, analog-to-digital (ND)and digital-to-analog (D/A) converters may be employed. The integratedcircuit comprises a predistortion circuit 304 which is coupled toreceive an input signal x(n) and generate an output signal z(n) whichcomprises a modified version of the input signal and is coupled to thepower amplifier. As set forth above, the predistortion circuit modifiesthe input signal to compensate for distortion in the power amplifiersuch that the output y(n) of the power amplifier correlates to the inputsignal. A characterization event for generating parameters to apply tothe predistortion circuit begins with a capture of a predeterminednumber L of samples of data from the input and output of the PA. Thesamples of the input are taken from the output of the predistortioncircuit. Accordingly, the output of the predistortion circuit and theoutput of the power amplifier are coupled to sample capture buffers 306.

Digital predistortion (DPD) involves performing the operations on adigital signal prior to digital-to-analog conversion. The samples fromthe output of the PA are typically obtained via some analog circuitryfollowed by an analog-to-digital converter, as will be described in moredetail in reference to FIG. 5. As will further be described in moredetail below, a trigger provided to the sample capture buffers enablesthe storing of the outputs of the predistortion circuit and the poweramplifier based upon a detected power value for the input signal. Thevalues stored in the sample capture buffers are provided to anestimation circuit 308. The estimation circuit will generate parameters“a” which will be applied to the predistortion circuit. The parametersmay be coefficients of a transfer function, for example, which modifythe input signal so that the output of the predistortion circuit offsetsthe distortion of the power amplifier. The new parameters “a” are storedin a parameters buffer 310 for use by the predistortion circuit. Thereare number of possible numerical techniques for generating theparameters. Discrete characterization events, where the coefficients arefound that best match the PA during some period of L samples duration,may be employed. By way of example, Least Mean Squares estimation overthe fixed block of L samples may be used. However, any other methodknown in the art for generating parameters for a predistortion circuitmay be used.

Turning now to FIG. 4, a block diagram of an integrated circuit having acircuit for reducing distortion at the output of a power amplifier andan embedded processor according to an embodiment of the presentinvention is shown. The integrated circuit 401 of FIG. 4 may bepartitioned into components that process a real-time digitally sampledsignal and components that relate to calculating the parameters “a”which may comprise estimation coefficients, for example. The real-timecomponents of FIG. 4 may comprise the predistortion circuit 402 andmemory elements 404, but may also include circuits for controlling thesecomponents, such as circuits applying a trigger signal to the samplecapture buffers. The memory elements 404 may comprise sample capturebuffers 406, parameter buffers 408, and data and control buffers 410.Communications between the real-time components are preferably made viadual-port random access memories (DPRAMs), which allow independentaccess to the memory contents from two sets of inputs and outputs. Anembedded processor 412 may be used to provide the estimation functionand generate the parameters for the predistortion circuit, which may beprovided to the parameter buffers 408 by a bus 414. The main workingmemory for the processor preferably comprises a dedicated embeddedmemory 416, as shown, and all the data that is manipulated by theembedded processor may pass through the bus. As will be described inmore detail below, multiple buses may be employed, depending upon thedata stored in the various memories.

The predistortion circuit 402 may be implemented in configurable logic,such as configurable logic blocks of the circuit of FIG. 1. Similarly,the dual port memories may be implemented as dual port random accessmemories, and may be implemented in the BRAMs of FIG. 1. Finally, theembedded processor 412 may be the processor 110 of FIG. 1, where theestimation circuit may be implemented in software running on theembedded processor. The embedded processor may be constructed fromavailable hardware resources, or may be implemented as a hardwareprimitive. By way of example, the embedded processor may be a hardwiredprocessor, such as a PowerPC processor, or may be a processorimplemented in configurable logic, such as a Microblaze processor, bothof which are available from Xilinx, Inc. of San Jose, Calif. andimplemented in Xilinx programmable logic devices.

An estimation sequence executed by the embedded processor will begin byreading and writing the Data and Control DPRAM to establish the triggerfor the data capture, with possible conditioning based of the state ofthe transmitted data. For example, it would be unwise to attempt togenerate estimation parameters during a time when there is no data beingtransmitted. After the L samples are captured in the capture buffers,these samples may then be processed by the estimation software runningon the embedded processor to produce the parameters, which are thenwritten into the parameters buffers. After the sample captures areprocessed, the sample capture buffers are available to the integratedcircuit, and particularly the embedded processor for use as furtherworking memory. This availability of memory after generating theparameters helps with overall hardware efficiency. In a practicalsystem, other functions such as measurements of the transmitted signalmay also be used by the estimation circuit, as will be described in moredetail below in reference to FIG. 6. These real-time functions areincluded in the predistortion circuit, and the data and control memoriesare provided for the estimation circuit to enable these functions.

Turning now to FIG. 5, a block diagram of an integrated circuit having acircuit for reducing distortion at the output of a power amplifier andan embedded processor according to an alternate embodiment of thepresent invention is shown. In particular, the integrated circuit 501comprises a baseband interface 502 coupled to receive an input signal,the output of which is coupled to a DUC/CFR circuit 504. The DUC/CFRcircuit 504 provides digital up conversion and crest factor reduction.Digital up conversion converts the input signal to the appropriatedigital format, while crest factor reduction helps to reduce thepeak-to-peak average power ratio, as is well known in the art. Theoutput of the DUC/CFR circuit 504 is applied to the predistortioncircuit 304, the output of which is coupled to both a digital-to-analogconverter 506 and sample capture buffers 306. The sample capture buffersalso receive an output from an analog-to-digital converter 510. As willbe described in more detail below, an embedded processor 520 controlsthe sample capture buffers to ensure that the correct parameters areprovided to the predistortion circuit. For example, the embeddedprocessor ensures that the received samples are brought into alignmentwith the transmitted samples with respect to frequency, time andamplitude. In addition to the amplitude of the signals being aligned,any delay between the two signals will be eliminated to ensure that theyare aligned in time. That is, because of the time required to passthrough the power amplifier will cause the signals to be out ofalignment, it is necessary to adjust the alignment of the signals.Finally, the delay aligned signal may be coupled to a frequency offsetcorrection circuit to ensure that the frequencies of the signal are thesame. This processing may be a part of the estimation circuit, and moreparticularly, a set of functions in the software running on the embeddedprocessor. The processing may be done in real time as they are received,or after they are stored.

The embedded processor 520 may be coupled to a plurality of buses. Forexample, a first bus 522 may be used to transfer data between the samplecapture buffers 512 and the embedded processor 520. A separate bus 524may be used to couple data between the embedded processor and a code anddata memory 526. While a single bus may be used, one benefit of usingmultiple buses is that the data may be transferred at different datarates. For example, the data from a sample capture buffers may takeadditional clock cycles to be received because they are received fromthe programmable logic, while the data from the code and data memory 522may be accessed directly by the embedded processor. Other circuits, suchas a floating point unit (FPU) 528, may be coupled to the embeddedprocessor. Similarly, a host/external interface 530 and a separate SRAM532 may be used to provide data to the code and data memory 526. Thecode and data memory 526 may contain any necessary computer code or datarequired to implement the predistortion circuit using the embeddedprocessor.

Turning now to FIG. 6, a block diagram of an integrated circuit havingcircuits for adapting coefficients according to an embodiment of thepresent invention is shown. One problem faced in implementing a poweramplifier in a system is that the average power that is beingtransmitted may change over time. For example, in a cellularcommunications system, a variation in average power may result fromtraffic density or call load. When predistortion parameters arecalculated at one average power, the correction of the distortion maynot be as beneficial when the transmitted signal changes to a differentaverage power. Particularly, spectral compliance for adjacent channelpower may not be achieved at the new power. According to one aspect ofthe invention, the circuit of FIG. 6 introduces a learning process whichtracks the characteristics of the power amplifier, and which operatesconcurrently with a look-up function that applies different parametersin response to power changes. While the learning process involves thecomputationally intensive parameter estimation, only the look-upprocess, which is computationally simple, needs to track the powervariations. Thus the computational throughput requirement is reduced,and a more compact integrated digital predistortion arrangement may beachieved.

In particular, the circuit of FIG. 3 has been modified as shown in FIG.6 to include circuits for implementing a first process, designated as aProcess A, for providing a lookup method for parameters which areapplied to the predistortion circuit, and a second process, designatedas a Process B, for updating the parameters. According to the embodimentof FIG. 6, a parameter sets look-up memory 604 is coupled to receive apower value, such as an average power of the input signal, from a powermeasurement circuit 606 to determine what values to apply to thepredistortion circuit. The power measurement circuit may be controlledby an interval timer 608, for example, which determines the period ofoperation for Process A. That is, new parameters may be loaded into theparameter buffers on a periodic basis according to an output of theinterval timer. The average power may be continuously and repeatedlymeasured over a time interval that is long enough to be representativeof the statistics that determine the spectral character of the signal,such as average power, but short enough to track the call-load. The timeinterval for determining the average power may be calculated on an orderof 10's of milliseconds, for example.

The parameters sets look-up memory contains N parameters sets withinformation tags, where N is a configuration variable greater than orequal to 1. The information tags may comprise, for example, metrics ofthe signal, such as an average power or the direction of the power ofthe signal, metrics of the capture that were used, such as a maximumindex or number of peaks, or time, such as interval ticks. In responseto the determination of a new power value, which may be generated onceper interval, the parameters sets look-up memory updates the parametersin the parameter buffer used by the predistortion circuit. As will bedescribed in more detail below, the parameters sets look-up memory rulesmay select a parameter set of the plurality of parameter sets accordingto range criteria. That is, a parameter set may represent a range ofpower values, and one parameter set may correspond to a given powermeasurement signal or other metric of the signal

The parameter sets may be updated according to the Process B controlledby a parameters update controller 610. In particular, an estimationcircuit 612 is coupled to receive the outputs of the sample capturebuffers and generate the parameters, as set forth above. The parametersupdate controller is responsible for triggering the estimation processand over-writing parameter sets and information tags according to itsdefined rules. The parameters update controller rules are preferablyalso based on the power measurements. The parameters sets look-up memoryand parameters update controller rules are driven by the characteristicsof a PA when predistorted (i.e. specifically what happens when thesignal power changes after the coefficients are calculated).

While Process B is also repetitive, it does not necessarily have atimer. Its period may be determined by the time taken for estimation,thereby improving hardware efficiency of the circuit. Estimation timesof the order of seconds by the parameter updates controller areacceptable because the system is fully adaptive at the Process Ainterval rate once the parameters sets look-up memory is populated.Where a microprocessor is used for the estimation of parameters, theprocesses enabling predistortion, other than the power measurement whichmust involve real-time logic, may be implemented in software. Further,Process A and Process B could be concurrent tasks in a real-timeoperating system environment. Alternatively, with a single-threadprogram, Process B may be a loop and Process A may be an interruptservice routine, with the interrupt provided by the interval timer.While the circuits for implementing Process A and Process B have beenshown in relation to the circuit of FIG. 3 by way of example, thesecircuits may also be implemented in the embodiments of FIGS. 4 and 5.

Turning now to FIG. 7, a graph shows an example of adjacent channelpower as a function of transmitted power for a plurality of parametersets having coefficients for a predistortion circuit according to anembodiment of the present invention. The characteristics of FIG. 7 showa measure of spectral performance versus output power. A convenientmeasure of spectral performance is an adjacent channel power (ACP) whichis defined for the wireless transmission standard being used. A plot ofthe characteristics shows ACP versus transmitted power for variouspowers at which the predistortion parameters were estimated. A linemarked “SPEC” is shown to indicate the maximum adjacent channel power tobe allowed by a given data transmission specification. As can be seen,at moderate and high powers, the predistortion performance only meetsthe specification in the vicinity of the power, designated as Pmax, P1and P2, at which the parameters were estimated. At lower transmissionpower levels, the predistortion performance is compromised at powersabove the power at which the parameters were estimated, but is generallyno worse below. It can be seen that if three parameter sets, designatedaccording to the transmitted power Pmax, P1, P2, were to be stored inthe parameters sets look-up memory, compliance would be assured if theset that was most appropriate to the transmitted power were to beselected by Process A. By way of example, the parameters sets look-upmemory rules select the P2 parameter set for powers below P_j2, the P1parameter set for powers between P_j2 and P_j1 and the Pmax parameterset for powers above P_j1 as shown by the dotted line. The rules needalso to account for an initial period of time before which some or allparameter sets have been obtained. Preferably, an initial parameter setbased upon the maximum power value for an input signal could be used.

Ideally, the parameters update controller rules would trigger anestimation function and store the parameters whenever Pmax, P1 and P2are encountered. While the power is continually varying with some degreeof randomness, it is never guaranteed that those powers will beencountered in a reasonable period of time. Accordingly, tolerances orvariation values, designated as Δ, are introduced. For example, the Pmaxset will be estimated whenever the power is between Pmax—Δ and Pmax,where Δ is a configuration variable. A Δ may be defined separately foreach parameter set, such as Δ max, A 1 and A 2 for parameter set Pmax,P1 and P2, respectively. The variation values are used to improve theinitial learning performance, and are initially set wide. The variationvalues may also be dynamically changed depending on the state of theparameters sets look-up memory, such as how many sets have beenestimated and at which powers. As time evolves and power measurementscloser to PMAX, P1 and P2 are encountered, the tolerances may be reducedto a specified minimum. As will be described in more detail below, thetolerance may be reduced to a minimum value after the detected powerfalls within a range for one of the parameter sets, and that parameterset is updated. It should be noted that ranges defined by the powervalues and variation values may overlap, in which case multipleparameter sets may be updated.

The tolerances may also be manipulated to accommodate for variations inradio frequency (RF) and PA gain. Variations in RF portions of thecircuit for transmitting the signal to the power amplifier and in thepower amplifier itself are generally slow variations. These variationsmay be related to changes in the thermal environment, and may be on atime-scale of minutes, for example. Because the performance of thepredistortion circuit may be sensitive to gain, it is desirable thatparameters are re-estimated unconditionally within a given time period.This re-estimation may be achieved by increasing the Δ's with time at aselected rate. Further, some PA's will have a Pmax characteristicsimilar to the P2 characteristic. That is, the parameters at Pmax aregood for all powers. In such a case, the parameters sets look-up memorymay only comprise a single parameter set, and the appropriate parametersupdate controller rule is to re-estimate the parameters whenever thepower is greater than the last power at which estimation was performed.The elements of FIGS. 3-6 may be implemented using the device of FIGS. 1and 2, where the real-time components may be constructed from thehardware resources available within an FPGA which may include logiccells and dedicated arithmetic and memory blocks, and the remainingcomponents may be implemented using the embedded processor as set forthabove.

When determining parameters for a predistortion circuit, a problemoccurs when the transmitted signal has a pulsed envelope, which istypical in high-speed data communications. If the data capture is madeduring the low period of the signal, the predistortion parameters willnot be useful during the high periods of the signal because the range ofdata is insufficient. For good predistortion estimation, the captureneeds to occur in the high period of the signal. The success ofpredistortion estimation is based on spectral emissions improvement.Accordingly, a statistical view of the spectral emissions is preferablyused because a measurement interval must be defined for a spectrum. Inpractice, this may be related to the sweep time of the spectrum analyzerused for standards compliance testing. The estimation is based ondiscrete characterization events, where the parameters are estimatedwith a capture of L samples of data from the input and output of the PA.In practice, the period for collecting the L samples will be muchshorter that the interval over which the spectral measurements aretaken. The success of the estimation will depend on these short durationcharacterization events being sufficiently accurate for the longerinterval. The circuits and methods of the present invention ensure thatthe L samples are statistically representative of the signal over alonger measurement interval.

Average power and amplitude histograms are continuously and repeatedlymeasured over a time interval that is long enough to be representativeof the statistics that determine the spectral character of the signal.This interval will need to be of the order of 10's of milliseconds.Typically the minimum spectrum analyzer sweep time may be approximately100 milliseconds and the maximum length of L samples may beapproximately 20 microseconds. As shown in FIG. 8, a graph indicatesthat the input signals is captured over a period of time known as theMean Signal Interval, where a typical 10 milliseconds interval is shown.The average power is the accumulation of |x(n)|² over the Mean SignalInterval. As will be described in more detail below, a histogram iscreated which comprises an accumulation of the number of occurrences ofeach of the discrete values of the signal amplitude |x(n)| over the MeanSignal Interval.

Turning now to FIG. 9, a block diagram of a circuit for accepting asample of an input signal to be used to calculate parameters for apredistortion circuit of an integrated circuit 902 is shown. The circuitof FIG. 3 has been modified to include a parameter sets look-up memory904 which is coupled to receive updated parameters from a controlcircuit 906. The control circuit is coupled to receive data related tothe information signal. In particular, a histogram 908 is created basedupon data from a power measurement circuit 910 over the accumulationinterval. The data may be used by a sample capture acceptance circuit912 based upon a signal from an accumulation interval circuit 914.

Data related to a sample of the input signal may also be stored in ahistogram 916 based upon an output of a power measurement circuit 918 inresponse to a control signal by a sample period circuit 920. As will bedescribed in more detail below, it is the data in the histogram 916taken over a short period of time that is compared against data in thehistogram 908 taken over a long period of time to determine whether thedata associated with the sample is acceptable data to be used togenerate coefficients for the predistortion circuit. A parameter updatecontroller 922 is coupled to receive the output of the sample captureacceptance circuit 912. The parameter update controller 922 is alsocoupled to an estimation circuit 924, which generates updatedcoefficients as described above and stored in the parameter sets look-upmemory 904. The control circuit 906 may comprise a microprocessorrunning software for performing the sample capture acceptance function,which will be described in detail below in reference to FIGS. 18 and 19.

The input signal detected by the power measurement circuit may berepresented in 16 bits, so that the amplitude will be a 15 bit number.In order to reduce the storage requirements, the amplitude may betruncated to N bits. By way of example, N=8 may be sufficient, resultingin 256 amplitudes which may be represented. According to one embodiment,the histogram circuit may comprise logic to initialize the RAM at thebeginning of each measurement interval and to transfer the content to abuffer at the end of each measurement interval. When a capture of theinput signal is triggered, the power and histogram are also computedover the interval of the capture, resulting in L samples. Acceptancecriteria are then applied to the captured samples over the captureinterval. As will be described in more detail below, the acceptancecriteria may be based on a comparison of data derived from the power andhistogram measurements over the capture interval with data derived fromthe power and histogram measurements over the Mean Signal Interval. Ifthe criteria are met as will be described in more detail below, thecaptured data is used for predistortion parameter estimation. If not,the capture is repeated. An upper limit on attempts to capture anacceptable sample to be used to generate parameters may be set as asystem error threshold. That is, if acceptable data for generatingpredistortion parameters is not received with a predetermined period oftime or within a predetermined number of attempts to acquire acceptabledata, an error flag may be generated. While the circuit of FIG. 9 showmodifications to the circuit of FIG. 3, the circuit may also includeelements of the other embodiments set forth above. Although thedescriptions of FIGS. 8 and 9 relate to power measurements according toone embodiment, the circuit of FIG. 9 could measure any of theinformation tags set forth above, or some other metric for the inputsignal.

According to one embodiment, each signal amplitude value addresses astorage location of a RAM, and data for that signal amplitude value isincremented every time that amplitude value occurs to create a count foreach given signal amplitude value. The RAM preferably comprises a dualport RAM, enabling separately accessing the data using two ports, aswill be described in detail. As shown in FIG. 10, the histogram 908comprises a random access memory for storing data related to the inputsignal over a period of time. The output of the power measurementcircuit 910 may be coupled to a truncation circuit 1004. The truncationcircuit is used for truncating the detected power value to reduce thenumber of storage locations required to store the data. For example, bytruncating the signal output by the power measurement circuit to eightbits, only two hundred fifty six storage locations would be required tostore a count for each of the two hundred fifty six power levels definedby the eight bits. A logic circuit 1006 provides an active write enablesignal to a first port, Port A, for the histogram 908 by generating anactive write enable signal associated with the accumulation interval.The data in a given address based upon the power measurement circuit isincremented by an incrementing circuit 1008 at the data output.Accordingly, data stored at a given address will be the previouslystored data incremented by 1 whenever data in that address is accessed.

Data may also be read out of the memory and reset as necessary. Forexample, a logic circuit 1010 is coupled to a write enable input for asecond port, Port B, of the RAM. After all of the data has been writtento the histogram 908 during the accumulation interval, that data may beread out of the memory based upon an address provided by address logiccircuit 1012 and stored in an accumulation data histogram values buffer1014. That is, the data that is transferred to buffer 1014 may be usedby the control circuit 906 to generate parameters for the predistortioncircuit, while new data is stored in the histogram 908. The valuesrepresenting the counts for the histogram may be reset by loading zeroesin the memory. Because the values stored in the memory represent acount, they must be reset. Accordingly, after the complete histogram hasbeen created during an accumulation interval, the values stored in thememory representing the counts for the different power levels are allreset to zero. The address logic circuit 1012 will allow reading thedata from the histogram 908 to fill the accumulation data histogramvalues buffer 1014, or allow resetting all of the memory locations tozero by sequencing through all of the addresses. Accordingly, the logiccircuit 1006 will generate a write enable signal to enable generatingthe count by loading new data into the memory during the accumulationinterval, while logic circuit 1010 will generate a write enable signalto enable writing to Port B after the accumulation interval to reset thehistogram.

The block diagram of FIG. 11 shows a random access memory for storingdata related to signals taken during a sample capture period whichoperates very similarly, but are based upon signals captured in responsea capture trigger signal. In particular, the histogram 916 comprises arandom access memory for storing data related to the input signal overthe sample period defined by the sample period circuit 920, which may be10 microseconds, for example. The output of the power measurementcircuit 918 may also be coupled to a truncation circuit 1104 to reducethe storage requirements as set forth above. A logic circuit 1106generates an active write enable signal to a Port A for the histogram916 by generating an active write enable signal for the sample period togenerate L samples. The data in a given address based upon a valuedetermined by the power measurement circuit is incremented by anincrementing circuit 1108 at the data output, as set forth above.

Data may also be read out of the memory and reset as necessary. Forexample, a logic circuit 1110 is coupled to a write enable input for aPort B of the circuit. After all of the data has been written to thehistogram 916 during the sample period, that data may be read out of thememory and stored in an sample data histogram values buffer 1114. Thevalues representing the counts for the histogram 916 may also be resetby loading zeroes in the memory. That is, after the values for histogram916 have been created during the sample period and stored in the sampledata histogram values buffer 1114, the values stored in the memoryrepresenting the counts for the different power levels of the L samplesare also all reset to zero. The logic circuit 1112 will allow readingthe data from the histogram 916 to fill the sample data histogram valuesbuffer 1114, or providing the addresses for resetting all of the memorylocations to zero. The values stored in the sample data histogram valuesbuffer 1114 and the values stored in the accumulation data histogramvalues buffer 1014 may then be used by the control circuit 906 todetermine whether an input signal is acceptable for generatingcoefficients for the predistortion circuit. It should be noted that thebuffers may not be needed as it may not be required to simultaneouslyprocess the histogram while getting the data for the next histogram.Because the captures are discrete one-shot events, there may be no needto perform another capture until finished processing the acceptancecriteria for the current one.

Turning now to FIG. 12, a block diagram of a circuit for generatingparameters for a predistortion circuit according to an embodiment of thepresent invention is shown. In particular, the circuit of FIG. 9 hasbeen amended to include a signal correction circuit 1202. As will bedescribed in more detail below, signals coupled to the sample capturebuffers will be adjusted to have the same amplitude and be aligned intime and frequency. In order for an estimation of parameters for apredistortion circuit to be successful, the amplitude of captured signalsamples must be the same and the signals aligned before parameterestimation is performed with the data. According to one aspect of theinvention, frequency offset correction is provided to the processingchain prior to estimation. Frequency offset correction will improve thequality of the estimation, which may lead to improved spectralcorrection.

As shown in the block diagram of FIG. 13, the signal correction circuit1202 of FIG. 12 comprises an amplitude alignment circuit 1302 coupled toreceive both the output of the predistortion circuit z(n) and the outputof the power amplifier y(n). The output of the amplitude alignmentcircuit comprises an amplitude adjusted output y′(n) of power amplifierwhich is coupled to a delay alignment circuit 1304. The delay alignmentcircuit will eliminate any delay between the two signals to ensure thatthey are aligned in time. That is, because of the time required to passthrough the power amplifier will cause the signals to be out ofalignment, it is necessary to adjust the alignment of the signals.Finally, the delay aligned signal y″(n) output by the delay alignmentcircuit 1304 will be coupled to a frequency offset correction circuit1306. Details for the various modifications will be described in moredetail below. In particular, amplitude alignment will be described inmore detail in reference to FIG. 14, delay alignment will be describedin more detail in reference to FIGS. 15 and 16, and frequency offsetcontrol will be described in more detail in reference to FIG. 17.

Turning now to FIG. 14, a block diagram of the amplitude alignmentcircuit 1302 of FIG. 13 according to an embodiment of the presentinvention is shown. A comparator 1402 is coupled to receive the outputof the predistortion circuit and the output of the power amplifier, andto generate an amplitude control signal based upon a comparison of thetwo signals. For example, the amplitude control signal may comprise botha magnitude and a direction. Assuming that the output of the poweramplifier y(n) is to be adjusted in amplitude, if the amplitude of y(n)is greater than z(n), the magnitude of y(n) is reduced by apredetermined magnitude determined by the comparator. The output y′(n)of the amplitude control circuit 1404 may be continuously fed back tothe comparator to provide a feedback loop to ensure that the amplitudeis maintained.

Turning now to FIG. 15, a block diagram of the delay alignment circuit1304 of FIG. 3 according to an embodiment of the present invention isshown. In particular, an edge detector 1502 is provided to generate adelay control signal to control a delay circuit 1504. The input signaly′(n) is coupled to a plurality of delay elements 1508. A multiplexer1510 is coupled to select the output after a desired number of delayelements of the plurality of delay elements 1508 based upon a delaycontrol signal as shown. The plurality of delay elements 1508 may be a64 tap delay line, for example. The delay elements of FIG. 15 may beimplemented in I/O blocks of the PLD described above with respect toFIG. 1. While the delay element of FIG. 15 is shown by way of example,other delay elements or configurations may be implemented.

Turning now to FIG. 16, a block diagram of the edge detection circuit1502 of FIG. 15 according to an embodiment of the present invention isshown. The edge detector of FIG. 16 comprises a level mode phasedetector that generates a binary output signal identifying the directionthe signal y′(n) has to change to be aligned with z(n). An initial delaycontrol signal will be based upon the y′(n) signal, and additionalcorrections will be based upon the y″(n) signal which is continuouslyprovided to the edge detector 1502. As shown in FIG. 16, the edgedetector 1502 includes edge detector 1602 coupled to receive z(n) and anedge detector 1604 coupled to receive y′(n) and y″(n), the output ofeach of which is coupled to a lead-lag state machine 1606. Edge detector1502 generates a delay control signal. When the y′(n) signal is leadingthe z(n) signal, a delay control signal is generated to provideadditional delay to the y′(n) signal. Conversely, when y′(n) signal islagging the z(n) signal, a delay control signal is generated to reducethe delay. While the circuits of FIGS. 13-16 are shown by way ofexample, other circuits or methods could be employed to adjust theamplitude and eliminate any delay between the output of thepredistortion circuit and the output of the power amplifier.

Turning now to FIG. 17, a block diagram of the frequency offsetcorrection circuit 1306 of FIG. 13 according to an embodiment of thepresent invention is shown. When mixing the PA output signal down to therequired baseband frequency to be compared with the output of thepredistortion circuit, some error in frequency may be introduced. Whilethe accuracy of the conversion may be very good (e.g. withinparts-per-million), the residual error may still affect predistortionperformance. Accordingly, frequency offset correction is performed,resulting in an improvement in the predistortion performance.

While the frequency offset may be corrected in a number of ways, thefrequency offset may corrected according to the present invention usingvarious approximations to simplify the calculation of a requiredfrequency. According to one embodiment, frequency offset correction isperformed by adjusting the phase of the received samples linearly inproportion with time, where the slope represents the frequency. Theslope can be estimated by standard linear regression of the phasedifference between the transmitted and received samples. Because thefrequency offset is small, the computation can be simplified. Moreparticularly, with x(n)=|x(n)|e^(JΦ(n)) and y(n)=|z(n)|e^(jΦ(n)), thesimplified frequency offset correction to y(n) involves multiplicationby a vector “a+bn”, where a and b are the linear regression coefficientsthat fit the data Δ(n)=e^(jΘ(n)-Φ(n))=(x(n)/|x(n)|)/(y(n)/|y(n)|).

The signals may be represented as complex numbers, where z(n)=a(n)+jb(n)and y(n)=c(n)+jd(n). Because the phase angle of the signals isrepresented by z(n)=|z(n)|·e^(jΘ(n)) and y(n)=|y(n)|·e^(jΦ(n)),

${{\overset{\bigvee}{z}(n)} = {{\mathbb{e}}^{j\;{\Theta{(n)}}} = {\frac{z(n)}{{z(n)}} = {\frac{a(n)}{\sqrt{( a^{2} ) + ( b^{2} )}} + {j\frac{b(n)}{\sqrt{( a^{2} ) + ( b^{2} )}}}}}}},{and}$${\hat{y}(n)} = {{\mathbb{e}}^{j\;{\Phi{(n)}}} = {\frac{y(n)}{{y(n)}} = {\frac{c(n)}{\sqrt{( c^{2} ) + ( d^{2} )}} + {j\frac{d(n)}{\sqrt{( c^{2} ) + ( d^{2} )}}}}}}$In order to correct y(n) to be frequency aligned with z(n), y(n) ismultiplied by Δ(n), such that y(n)·Δ(n)=z(n). In order to simplify thecalculation, the Δ(n) is approximated as a linear equation representing{hacek over (z)}(n)/ŷ(n). Accordingly, Δ(n) may be represented as alinear equation A+nB corresponding to {hacek over (z)}(n)/ŷ(n), where Acomprises an offset and B comprises a slope. That is, because the anglerepresenting the phase difference should be small, Δ(n)=e^(jΘ(n)-Φ(n))may be represented by an approximation of {hacek over (z)}(n)/ŷ(n).Because this involves only a complex magnitude calculation and simplearithmetic, the computation involving trigonometric functions isavoided.

Turning now to FIG. 18, a flow chart shows criteria for determiningwhether a sample of an input signal is acceptable to calculateparameters for a predistortion circuit according to an embodiment of thepresent invention. The criteria may include, but need not be limited to,a mean power test, a peak amplitude test, a peak density test, and a lowpower test. The mean power test is used to verify that the mean power ofthe captured samples is greater than the mean power of the input signalover the Mean Signal Interval, within some tolerance. Accordingly, themean value of the power of the input signal detected during the sampleperiod and the mean value of the accumulated power for the input signalsover the accumulation interval are determined at a step 1802. It is thendetermined whether the mean value of the sampled signal of the inputsignal is greater than the mean value of the accumulated power takenover the accumulation period at a step 1804. If the mean value of thesampled signal is not greater than the mean value of the accumulatedpower, then the test fails. The mean value of the sampled signal mayhave to exceed the mean value of the accumulated power by apredetermined amount.

In the peak amplitude test, the ratio of the sum of the captured samplesoccurring with at least a given probability to the sum of the fullsignal measured over the accumulation interval should be greater than apredetermined value. That is, not only must the value of a given sampleto be used in the peak amplitude test be greater than a predeterminedvalue, but the value of a given sample must have occurred a certainnumber of times. For example, if one hundred samples are taken duringthe capture sample period, a minimum number of samples having a givenamplitude may be required for those samples to be used in the peakamplitude test. The Mean Signal Interval histogram is used to find atarget value for the peak amplitude k of the capture. By way of example,if Y is the cumulative sum of all histogram values of the capturedsample with index>k, then k is the value for which ratio of Y to thecumulative sum of the signal over the accumulation interval is p. Forp=0, this comparison reduces to a simple comparison of peak values. Thealgorithm with non-zero p is designed to prevent statisticallyinsignificant individual peaks preventing acceptance. Accordingly, atarget value for a peak amplitude k is identified at a step 1806. It isthen determined whether the ratio of the cumulative sum of the samplesgreater than k and having a given probability to the cumulative sum ofthe input signal over the accumulation interval is greater than apredetermined value at a step 1808.

In the peak density test, the fraction of samples in the capturerelative to a defined amplitude threshold level should be greater thanthe fraction of samples of the full signal relative to the definedamplitude threshold level. The histograms are used to count the numberof samples greater than the threshold. Accordingly, the peak densitiesof the sample and the accumulated power measurements are determined at astep 1810. It is then determined whether the fraction of the peakdensity of the sample is greater than the fraction of the peak densityof the accumulated power measurements at a step 1812.

Finally, a test is performed to determine whether a predetermined numberof the L sample signals are below a predetermined threshold, which wouldindicate that the signal is not acceptable. That is, even if the signalmay otherwise meet the criteria of the three previous tests, the signalis still tested to determine whether a sufficient number of the L samplevalues are low signals (i.e. a minimum power value of either zero orbelow a predetermined threshold). Accordingly, the number of samples ofthe captured samples having minimum power values is determined at a step1814. It is then determined whether the number of samples having minimumpower values has exceeded a predetermined value at a step 1816. Althoughthe flow chart of FIG. 18 indicates a failure during the determinationof an adequate sample for generating parameters if any one of the testsfails, the method could indicate a failure if a certain percentage oftests fail or if a certain combination of tests fail. While the criteriaof FIG. 18 are shown by way of example, other tests could be used.

Turning now to FIG. 19, a flow chart shows a method of determiningwhether a sample of an input signal is acceptable to be used tocalculate parameters for a predistortion circuit according to anembodiment the present invention. Criteria for determining whether dataassociated with the input signal is acceptable to be used to calculateparameters for a predistortion circuit are established at a step 1902.The criteria could be those set forth above in FIG. 18, or othersuitable criteria. Power measurements for the input signal areaccumulated over an accumulation period at a step 1904. Powermeasurements for a sample of the input signal are taken in response to acapture trigger at a step 1906. A count associated with amplitudes isgenerated by incrementing a current value of data stored in the addresswhen the address is accessed at a step 1908. Information related to theinput signal is stored in a memory, wherein detected amplitudes of theinput signal for the accumulated power measurements are used asaddresses for storing counts associated with the detected amplitudes ata step 1910. The power measurements for the input signal taken inresponse to the capture trigger are compared to the accumulated powermeasurements at a step 1912. It is then determined whether multipletests established to determine whether the sample of the input signal isacceptable to be used to calculate parameters for a predistortioncircuit are required at a step 1914. If so, it is also determinedwhether the signal is required to pass all of the tests to be acceptableat a step 1916. If so, it is then determined whether all of theplurality of tests applied to the sample of the input signal are passedat a step 1918. Otherwise, it is determined whether any required testapplied to the sample of the input signal is passed at a step 1920.

Finally, turning now to FIG. 20, a flow chart shows a method ofgenerating parameters for a predistortion circuit according to anembodiment the present invention. In particular, an input signal to beamplified by a power amplifier is received at the predistortion circuitat a step 2002. An output of the power amplifier is received at an inputof the integrated circuit at a step 2004. An output of the predistortioncircuit is compared with the output of the power amplifier at a step2006. It is then determined whether there is a difference between theamplitude of the output of the predistortion circuit and the output ofthe power amplifier at a step 2008. If so, the amplitude of the outputof the predistortion circuit or the output of the power amplifier isadjusted to make the amplitude of the predistortion circuit and theoutput of the power amplifier equal at a step 2010. It is thendetermined whether there is any delay between the output of thepredistortion circuit and the output of the power amplifier at a step2012. If so, any delay between the output of the predistortion circuitand the output of the power amplifier is eliminated at a step 2014. Itis also determined whether there is any difference in the frequencies ofthe output of the predistortion circuit with the output of the poweramplifier at a step 2016. If so, a phase difference between the outputof the predistortion circuit and the output of the power amplifier isdetermined at a step 2018. An approximation of the phase difference isthen generated by converting a trigonometric representation of the phasedifference to an algebraic representation of the phase difference at astep 2020, and the frequency offset between the outputs of thepredistortion circuit and the output of the power amplifier iseliminated at a step 2022. The parameters to be applied to thepredistortion circuit are generated based upon the output of thepredistortion circuit and the output of the power amplifier for whichany differences in amplitude, delay or frequency have been eliminated ata step 2024. The parameters to be applied to the predistortion circuitin a look-up memory are stored at a step 2026. Parameters to be appliedto the predistortion circuit are selected based upon a power measurementof the input signal at a step 2028. The method of FIG. 20 may beimplemented using any of the circuits as shown and described above, orusing other suitable circuits. While adjustments as shown are made tothe output of the power amplifier, the adjustments could be made to theoutput of the predistortion circuit to conform the two signals.

It can therefore be appreciated that the new and novel circuit for andmethod of reducing distortion at the output of a power amplifier hasbeen described. It will be appreciated by those skilled in the art thatnumerous alternatives and equivalents will be seen to exist whichincorporate the disclosed invention. As a result, the invention is notto be limited by the foregoing embodiments, but only by the followingclaims.

1. A method of accepting a sample of an input signal to be used tocalculate parameters for a predistortion circuit in an integratedcircuit, the method comprising: accumulating data associated with aninput signal over a period of time; detecting the sample of the inputsignal at a predetermined time; comparing the sample of the input signalto the accumulated data; and determining whether the sample of the inputsignal is acceptable to be used to calculate parameters for thepredistortion circuit.
 2. The method of claim 1 further comprisingestablishing criteria for determining whether the sample of the inputsignal is acceptable to be used to calculate parameters for thepredistortion circuit, wherein the criteria comprises a plurality oftests.
 3. The method of claim 2 wherein determining whether the sampleof the input signal is acceptable comprises determining whether at leastone test of the plurality of tests applied to the sample of the inputsignal is passed.
 4. The method of claim 2 wherein determining whetherthe sample of the input signal is acceptable comprises determiningwhether all tests of the plurality of tests applied to the sample of theinput signal are passed.
 5. The method of claim 1 wherein accumulatingdata associated with an input signal over a period of time comprisesstoring information related to the input signal in a memory, whereindetected amplitudes of the input signal are used as addresses forstoring counts associated with the detected amplitudes.
 6. The method ofclaim 5 wherein a count associated with an amplitude is generated byincrementing a current value of data stored in the address when theaddress is accessed.
 7. The method of claim 6 further comprisinggenerating a calculated value based upon the accumulated data, whereincomparing the detected data associated with the input signal to theaccumulated data comprises comparing the detected data to the calculatedvalue.
 8. A method of accepting a sample of an input signal to be usedto calculate parameters for a predistortion circuit in an integratedcircuit, the method comprising: establishing criteria for determiningwhether data associated with the input signal is acceptable to be usedto calculate parameters for the predistortion circuit; accumulatingpower measurements for the input signal over a period of time; taking apower measurement for a sample of the input signal in response to acapture trigger; comparing the power measurement for the input signaltaken in response to the capture trigger to the accumulated powermeasurements; and determining whether the sample of the input signal isacceptable to be used to calculate parameters for the predistortioncircuit.
 9. The method of claim 8 wherein determining whether the sampleof the input signal is acceptable comprises determining whether theratio of the cumulative sum of samples greater than k and having a givenprobability to the cumulative sum of the input signal over the period oftime is greater than a predetermined value.
 10. The method of claim 8wherein determining whether the sample of the input signal is acceptablecomprises determining whether the average power of the captured sampleis greater than the average power of the accumulated power measurementsfor the input signal.
 11. The method of claim 8 wherein determiningwhether the sample of the input signal is acceptable comprisesdetermining whether the peak density of the sample is greater than thepeak density of the accumulated power measurements.
 12. The method ofclaim 8 wherein determining whether the sample of the input signal isacceptable comprises determining whether a number of samples having aminimal power value is below a predetermined threshold.
 13. The methodof claim 8 wherein determining whether the sample of the input signal isacceptable comprises determining whether at least one test of aplurality of tests is passed.
 14. The method of claim 8 whereindetermining whether the sample of the input signal is acceptablecomprises determining whether all tests of a plurality of tests arepassed.
 15. A circuit for accepting a sample of an input signal to beused to calculate parameters for a predistortion circuit in anintegrated circuit, the circuit comprising: a predistortion circuitcoupled to receive the input signal; a first memory for storing dataassociated with the input signal and accumulated over a period of time;a second memory storing data associated with a sample of the inputsignal; and a control circuit coupled to the first memory and the secondmemory, the control circuit comparing the data in the second memory tothe data associated with the input signal stored in the first memory fordetermining whether the sample of the input signal is acceptable to beused to calculate parameters for the predistortion circuit.
 16. Thecircuit of claim 15 wherein detected amplitudes of the input signal areused as addresses for storing a count associated with the detectedamplitudes of the first memory.
 17. The circuit of claim 15 furthercomprising an interval timer coupled to the first memory, wherein theinterval timer enables storing a plurality of power values over theperiod of time.
 18. The circuit of claim 15 wherein the control circuitgenerates a trigger signal to capture the sample of the input signal anda sample of an output of a power amplifier received at the integratedcircuit.
 19. The circuit of claim 18 further comprising sample capturebuffers storing an output of the predistortion circuit and the output ofthe power amplifier in response to the trigger signal.
 20. The circuitof claim 19 further comprising an estimator circuit coupled to thesample capture buffers, wherein the estimator circuit generatesparameters for the predistortion circuit based upon the output of thepredistortion circuit and the output of the power amplifier received atthe integrated circuit.